Such canisters are mass-produced articles and, owing to their method of production, the manufacturing tolerances to be expected from them are also not very high. They are used in widely differing environments which, for example, can be at -40 degrees C. to +75 degrees C. The force with which the screw cap is tightened also varies widely. When men are very tired and it is very cold, they have little strength to spare. It is not always important that emptying of the contents should proceed rapidly. Nevertheless, there are emergency situations in which rapid emptying of the canister is important and the ability to do this may be life-preserving. One need only consider a situation in which, for example, fuel has to be transferred from the canister to the tank on a swaying boat. If this takes too long, even a strong man cannot hold the canister balanced for long. If the transfer takes too long, there is the risk that the pouring spout will slip out of the inlet nozzle of the tank and the fuel will miss, with all the dangers that result from this, such as, for example, the risk of explosion. Frequently, it is also important to spill as little as possible, when the canister is the last spare canister, etc.
Referring to U.S. Pat. No. 4,928,861, the screw cap includes an annular channel (48) that opens away from the cap base (37), at least one through airhole (50), which leads through the cap base (37) from the annular channel (48), and an annular wall with an internal thread (34) that matches the external screw thread (18) of a pouring nozzle (16), the screw cap (32) being arranged to assume a particular angular position relative to the pouring nozzle when the screw cap is screwed fully onto the pouring nozzle.
A single through airhole 50 together with the annular channel (48) is usually sufficient. However, if this through airhole (50) is completely or partially blocked and/or if the annular channel is completely or partially closed, the air no longer flows back correctly and the abovementioned consequences can occur. For various reasons, the cross-section of the annular channel (48) can be reduced or completely closed in practice: for example enough viscous fluid may get into it at very low temperatures; or it is simply blocked; or its cross-section has been squashed shut by some other occurrence.